Voltage-controlled oscillator, radio communication apparatus and voltage-controlled oscillation method for reducing degradation of phase noise characteristic

ABSTRACT

A voltage-controlled oscillator having an inductor circuit, n pieces (n is two or more) of variable capacitance circuit having variable capacitance elements, negative resistance circuits, and reference voltage generation means of generating a reference voltage from a power supply voltage, and wherein a predetermined reference voltage is inputted to some terminals of the variable capacitance elements of the n pieces of variable capacitance circuit, a control voltage is inputted to the other terminals thereof, and of the variable capacitance elements of the n pieces of variable capacitance circuits, the predetermined reference voltage inputted to some terminals of the variable capacitance elements of at least two pieces of the variable capacitance circuit is different.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a voltage-controlled oscillator usedfor a radio communication device, a radio communication apparatus usingthe voltage-controlled oscillator and a voltage-controlled oscillationmethod.

2. Related Art of the Invention

A voltage-controlled oscillator is widely used as a means of generatinga local oscillation signal of a radio communication device.

FIG. 12 shows an example of a prior art configuration of such avoltage-controlled oscillator.

In FIG. 12, reference numerals 1 a and 1 b denote oscillationtransistors, 2 a and 2 b denote inductors, and 3 a and 3 b denotevariable capacitance elements. Reference numeral 4 denotes a powersupply terminal, 5 denotes a frequency control terminal, and 6 denotes acurrent source. A bias circuit and other elements are omitted in FIG.12.

In FIG. 12, the inductors 2 a and 2 b and variable capacitance elements3 a and 3 b constitute a parallel resonant circuit. As a capacitancevalue of the variable capacitance element changes due to a difference involtages at both ends thereof, the capacitance values of the variablecapacitance elements 3 a and 3 b change due to a control voltage appliedto the frequency control terminal 5, and consequently a resonancefrequency of the parallel resonant circuit changes.

As an oscillation frequency of the voltage-controlled oscillatoroscillates in the neighborhood of the resonance frequency of theresonant circuit, it is possible, by adjusting the control voltage, tocontrol the oscillation frequency of the voltage-controlled oscillatorto be a desired frequency. The oscillation transistors 1 a and 1 b areintended to generate negative resistance and cancel losses due to aparasitic resistance component of the resonant circuit so as to satisfyan oscillation requirement.

A relationship between the control voltage and the oscillation frequencyof the voltage-controlled oscillator is virtually determined by acharacteristic of the variable capacitance element. It is desirable thatthe variable capacitance element changes the capacitance slowly in awide range of the control voltage. It is because, in the case of a PLL(phase lock loop) by using the voltage-controlled oscillator, atransient response characteristic and a noise band characteristic of aPLL circuit depend on frequency sensitivity with respect to the controlvoltage. Therefore, if the frequency sensitivity is different accordingto the frequency, the characteristic of the PLL circuit itself changesaccording to the frequency. In an area where the frequency sensitivitywith respect to the control voltage is high, there is a problem that aphase noise characteristic is degraded because the frequency changes dueto slight noise caused on a frequency control terminal.

In reality, however, it is difficult to utilize the variable capacitanceelement of high linearity. The reason for this is that when thevoltage-controlled oscillator is formed on a semiconductor substrate,costs increase because of a special process needed to form the variablecapacitance element. FIG. 13( a) shows the variable capacitance elementutilizing a gate capacitance widely used in a CMOS process, and FIG. 13(b) shows the variation of the gate capacitance in the case of applyingthe reference voltage to the gate of a MOS transistor and applying thecontrol voltage to a drain-source side. Thus, in the case of thevariable capacitance element utilizing the gate capacitance of the MOStransistor generally used, the capacitance value abruptly changes in theneighborhood of a threshold voltage (Vth in the drawing). Thus, theoscillation frequency also abruptly changes in the neighborhood of athreshold. Consequently, there arises a problem that the transientresponse characteristic and noise band characteristic of the PLL circuitusing this VCO significantly change depending on the frequency.

The circuit described below has already been proposed in order to solvesuch problems.

FIG. 14 is a circuit showing a technique of improving the linearity ofthe variable capacitance element (refer to Japanese Patent Laid-Open No.2001-352218 for instance). In FIG. 14, the same portions as thosepreviously described are given the same symbols and a descriptionthereof will be omitted.

Reference numerals 10 a, 10 b, 11 a, 11 b, 12 a and 12 b denote thevariable capacitance elements, and 13 denotes a level shift circuit. Acontrol signal inputted from the frequency control terminal 5 isinputted to the level shift circuit 13, and voltages having shifted byVd such as Vt, Vt−Vd and Vt−2Vd are outputted from three outputterminals outputted from the above described level shift circuit. Inthis case, the characteristics of the variable capacitance elements (10a to 12 b) against the control voltage Vt are the characteristicsshifted by Vd as shown in FIG. 15. As the capacitance of the resonantcircuit is a total of these six capacities, a total capacitance thereofis the characteristic indicated by a dashed line in FIG. 15 (A in FIG.15) so that the change in the capacitance against the control voltagecan be moderate.

In the above method, the level shift circuit 13 is constituted by usingthe transistors such as FET as shown in FIG. 22. This is because thelevel shift circuit 13 requires high input impedance in order to hold aDC voltage inputted from the frequency control terminal 5.

According to the above method, it is possible to improve the phase noisecharacteristic of the control signal, but it is not possible to curb thevariation or the noise in the power supply voltage. To be more specific,a difference voltage between the power supply voltage and the controlsignal is applied to both ends of the variable capacitance elements 10a, 10 b, 11 a, 11 b, 12 a and 12 b, and so they are influenced by thevariation or the noise in the power supply voltage even if the noise onthe control signal side is curbed. Therefore, the voltage-controlledoscillator changes its oscillation frequency due to the influence ofminute variation or the noise in the power supply voltage.

As a countermeasure, there has been proposed a configuration wherein onevoltage applied to each variable capacitance element is not the powersupply voltage, and each variable capacitance element is interruptedfrom the power supply voltage with a blocking capacitor so as to supplythe reference voltage different from the power supply voltage (refer to“Prospects of CMOS Technology for High-Speed Optical CommunicationCircuits” by Behzad Razavi, IEEE Journal of Solid-State Circuit, vol.37, No. 9 September 2002, pp. 1135–1144 for instance).

SUMMARY OF THE INVENTION

According to the above method, however, a reference voltage is generatedby directly utilizing for example, a resistance-divided power supplyvoltage so that the reference voltage is influenced by variation ornoise in the power supply voltage after all.

A configuration of level-shifting a control voltage has a problem that aphase noise characteristic is degraded due to the noise generated from atransistor such as FET.

In consideration of the above problems, an object of the presentinvention is to provide a voltage-controlled oscillator capable ofcurbing influence of the variation or the noise in the power supplyvoltage, a radio communication apparatus using the voltage-controlledoscillator and a voltage-controlled oscillation method, or to providethe voltage-controlled oscillator, radio communication apparatus andvoltage-controlled oscillation method capable of curbing degradation ofthe phase noise characteristic of a control signal and/or the powersupply voltage.

The 1^(st) aspect of the present invention is a voltage-controlledoscillator having:

an inductor circuit having an inductor;

n pieces (n is two or more) of variable capacitance circuit having avariable capacitance element and having blocking capacitors ofinterrupting a direct current at both ends thereof, a negativeresistance circuit, and reference voltage generation means of generatinga reference voltage from a power supply voltage, and wherein:

said inductor circuit, said n pieces of variable capacitance circuit andsaid negative resistance circuit are connected in parallel;

a predetermined reference voltage is inputted to some terminals of thevariable capacitance elements of said n pieces of variable capacitancecircuit;

a control voltage of feedback-controlling an oscillation frequency isinputted to the other terminals of the variable capacitance elements ofsaid n pieces of variable capacitance circuits; and

of said n pieces of variable capacitance circuits, the predeterminedreference voltage inputted to one of the terminals of the variablecapacitance elements of at least two of said n pieces of variablecapacitance circuit is different.

The 2^(nd) aspect of the present invention is the voltage-controlledoscillator according to the 1^(st) aspect of the present invention,wherein, when said n pieces of variable capacitance circuits arearranged in decreasing order of reference voltages inputted to said npieces of variable capacitance circuits, a difference between thevoltage inputted to one terminal of the variable capacitance element ofan m-th (m is between 2 and n) variable capacitance circuit and thevoltage inputted to one terminal of the variable capacitance element ofan m−1-th variable capacitance circuit is Vd.

The 3^(rd) aspect of the present invention is the voltage-controlledoscillator according to the 1^(st) aspect of the present invention,wherein said reference voltage generation means has series-connected n+1pieces of resistor, and said n pieces of reference voltage are generatedby having said power supply voltage divided by said series-connected n+1pieces of resistor.

The 4^(th) aspect of the present invention is the voltage-controlledoscillator according to the 1^(st) aspect of the present invention,wherein said reference voltage generation means has series-connected oneresistor and n pieces of diode, and said n pieces of reference voltageare generated by having said power supply voltage divided by saidseries-connected one resistance and n pieces of diode.

The 5^(th) aspect of the present invention is the voltage-controlledoscillator according to the 4^(th) aspect of the present invention,wherein said diodes are formed by transistors having bases andcollectors thereof shorted.

The 6^(th) aspect of the present invention is the voltage-controlledoscillator according to the 3^(rd) aspect of the present invention,wherein said reference voltage generation means further has an activefilter.

The 7^(th) aspect of the present invention is the voltage-controlledoscillator according to the 1^(st) aspect of the present invention,wherein said reference voltage generation means has series-connected oneactive filter and n pieces of resistors, and said n pieces of referencevoltage are generated by having said power supply voltage divided bysaid series-connected one active filter and n pieces of resistors.

The 8^(th) aspect of the present invention is the voltage-controlledoscillator according to the 1^(st) aspect of the present invention,wherein output sides of outputting said reference voltages of saidreference voltage generation means are grounded via capacitive elementsrespectively.

The 9^(th) aspect of the present invention is the voltage-controlledoscillator according to the 1^(st) aspect of the present invention,further having connected in parallel thereto a frequency band settingvariable capacitance circuit having a variable capacitance element,having a power supply voltage inputted to one terminal of said variablecapacitance element and having a voltage according to a frequency bandto be used of a plurality of voltages inputted to the other terminal ofsaid variable capacitance element so as to change a capacitance of saidvariable capacitance element, wherein, when the capacitance of thevariable capacitance element of said first variable capacitance circuitbecomes less than a predetermined value, a variable range is reduced asto the capacitance of at least one variable capacitance circuit of saidn pieces of variable capacitance circuit.

The 10^(th) aspect of the present invention is the voltage-controlledoscillator according to the 9^(th) aspect of the present invention,wherein at least one variable capacitance circuit of said n pieces ofvariable capacitance circuit has the variable capacitance elementconnected in parallel to said variable capacitance element, and thevoltage according to a frequency to be used of said plurality ofvoltages is inputted to the other terminal of one of said variablecapacitance elements instead of said control voltage so that thevariable range is reduced as to the capacitance of at least one variablecapacitance circuit of said n pieces of variable capacitance circuitaccording to the reduction in the capacitance of the variablecapacitance element of said frequency band setting variable capacitancecircuit.

The 11^(th) aspect of the present invention is the voltage-controlledoscillator according to the 9^(th) aspect of the present invention,wherein said blocking capacitor is comprised of two or more capacitorsconnected via a switch, and said switch is off or on according to thefrequency to be used so that the capacitance of said blocking capacitorsis reduced and a variable range of the capacitance of at least one ofsaid n pieces of variable capacitance circuit is reduced.

The 12^(th) aspect of the present invention is the voltage-controlledoscillator according to the 1^(st) aspect of the present invention,wherein said variable capacitance element operates by utilizing a gatecapacitance of an MOS transistor.

The 13^(th) aspect of the present invention is a voltage-controlledoscillator having:

an inductor circuit having an inductor;

n pieces (n is two or more) of variable capacitance circuit having avariable capacitance element, and a negative resistance circuit, andwherein:

said inductor circuit, said n pieces of variable capacitance circuit andsaid negative resistance circuit are connected in parallel, and acontrol voltage of feedback-controlling an oscillation frequency isinputted to some terminals of the variable capacitance elements of saidn pieces of variable capacitance circuit; and

of said n pieces of variable capacitance circuits, capacitance variationcharacteristics against voltage of the variable capacitance elements ofat least two thereof are different.

The 14^(th) aspect of the present invention is the voltage-controlledoscillator according to the 13^(th) aspect of the present invention,wherein said variable capacitance elements of which capacitancevariation characteristics are different against the voltage areimplemented by MOS capacitance of different impurity concentrations.

The 15^(th) aspect of the present invention is the voltage-controlledoscillator according to the 13^(th) aspect of the present invention,wherein said variable capacitance elements of which capacitancevariation characteristics are different against the voltage areimplemented by the MOS capacitance utilizing a p-channel MOS and the MOScapacitance utilizing an n-channel MOS.

The 16^(th) aspect of the present invention is the voltage-controlledoscillator according to the 13^(th) aspect of the present invention,wherein said variable capacitance elements of which capacitancevariation characteristics are different against the voltage areimplemented by the MOS capacitance of which terminals are a gate and adrain-source and the MOS capacitance of which terminals are the gate anda well.

The 17^(th) aspect of the present invention is a radio communicationapparatus having a PLL circuit of outputting a signal of a targetfrequency having the voltage-controlled oscillator according to the1^(st) or the 13^(th) aspect of the present invention.

The 18^(th) aspect of the present invention is a voltage-controlledoscillation method of a voltage-controlled oscillator having:

an inductor circuit having an inductor;

n pieces (n is two or more) of variable capacitance circuit having avariable capacitance element and having blocking capacitors ofinterrupting a direct current at both ends thereof, a negativeresistance circuit, and reference voltage generation means of generatinga reference voltage from a power supply voltage, with said inductorcircuit, said n pieces of variable capacitance circuit and said negativeresistance circuit connected in parallel, and wherein there are:

a step of inputting a predetermined reference voltage to some terminalsof the variable capacitance elements of said n pieces of variablecapacitance circuit; and

a step of inputting a control voltage of feedback-controlling anoscillation frequency to the other terminals of the variable capacitanceelements of said n pieces of variable capacitance circuits, and

the predetermined reference voltage inputted to one of the terminals ofthe variable capacitance elements of at least two of said n pieces ofvariable capacitance circuit is different.

According to the present invention, it is possible to provide thevoltage-controlled oscillator capable of curbing influence of thevariation or the noise in the power supply voltage, the radiocommunication apparatus and the voltage-controlled oscillation methodusing the voltage-controlled oscillator, or to provide thevoltage-controlled oscillator, radio communication apparatus andvoltage-controlled oscillation method capable of curbing the degradationof the phase noise characteristic.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram showing a configuration of avoltage-controlled oscillator according to a first embodiment of thepresent invention;

FIG. 2( a) is a diagram for explaining a principle of operation of thevoltage-controlled oscillator according to the first embodiment of thepresent invention;

FIG. 2( b) is a diagram for explaining a principle of operation of thevoltage-controlled oscillator according to the first embodiment of thepresent invention;

FIG. 3 is a circuit diagram showing the configuration of referencevoltage generation means of the voltage-controlled oscillator accordingto the first embodiment of the present invention;

FIG. 4 is a circuit diagram showing the configuration of the referencevoltage generation means of the voltage-controlled oscillator accordingto the first embodiment of the present invention;

FIG. 5 is a circuit diagram showing the configuration of the referencevoltage generation means of the voltage-controlled oscillator accordingto the first embodiment of the present invention;

FIG. 6 is a circuit diagram showing the configuration of the referencevoltage generation means of the voltage-controlled oscillator accordingto the first embodiment of the present invention;

FIG. 7 is a circuit diagram showing the configuration of the referencevoltage generation means of the voltage-controlled oscillator accordingto the first embodiment of the present invention;

FIG. 8 is a circuit diagram showing the configuration of thevoltage-controlled oscillator according to a second embodiment of thepresent invention;

FIG. 9 is a circuit diagram showing the configuration of a variablecapacitance of the voltage-controlled oscillator according to the secondembodiment of the present invention;

FIG. 10( a) is a diagram for explaining a principle of thevoltage-controlled oscillator according to the second embodiment of thepresent invention;

FIG. 10( b) is a diagram for explaining a principle of thevoltage-controlled oscillator according to the second embodiment of thepresent invention;

FIG. 11 is a circuit diagram showing another configuration example ofthe variable capacitance of the voltage-controlled oscillator accordingto the second embodiment of the present invention;

FIG. 12 is a circuit diagram showing the configuration of thevoltage-controlled oscillator in the past;

FIG. 13( a) is diagram showing a variable capacitance element utilizinga gate capacitance widely used in a CMOS process;

FIG. 13( b) is a diagram showing change in a capacitance value against acontrol voltage of a MOS capacitance;

FIG. 14 is a circuit diagram showing the configuration of thevoltage-controlled oscillator in the past;

FIG. 15 is a diagram for explaining the principle of operation of thevoltage-controlled oscillator in the past;

FIG. 16 is a circuit diagram showing another configuration example ofthe voltage-controlled oscillator according to the first embodiment ofthe present invention;

FIG. 17 is a circuit diagram showing a configuration example of thevoltage-controlled oscillator according to a third embodiment of thepresent invention;

FIG. 18 is a diagram for explaining the principle of operation of thevoltage-controlled oscillator according to the third embodiment of thepresent invention;

FIG. 19 is a circuit diagram showing a configuration example of thevoltage-controlled oscillator according to the third embodiment of thepresent invention;

FIG. 20 is a circuit diagram showing a configuration example of thevoltage-controlled oscillator according to the third embodiment of thepresent invention;

FIG. 21 is a diagram for explaining the principle of operation of thevoltage-controlled oscillator according to the third embodiment of thepresent invention; and

FIG. 22 is a diagram showing the configuration of a level shift circuitof the voltage-controlled oscillator in the past.

DESCRIPTION OF SYMBOLS

-   100 Power source terminal-   101 current source-   102 Frequency control terminal 103 a, 103 b Oscillation transistors-   104 a, 104 b Inductors-   105 a, 105 b, 106 a, 106 b, 107 a, 107 b Variable capacitance    elements-   108 a, 108 b, 109 a, 109 b, 110 a, 110 b DC-cut capacitors-   111 a, 111 b, 112 a, 112 b, 113 a, 113 b High-frequency blocking    resistors-   114 Reference voltage generation means

PREFERRED EMBODIMENTS OF THE INVENTION

Hereafter, embodiments of the present invention will be described byreferring to the drawings.

(First Embodiment)

FIG. 1 shows a configuration of a voltage-controlled oscillatoraccording to a first embodiment of the present invention, in which abias circuit and so on is omitted.

In FIG. 1, reference numeral 100 denotes a power source terminal as anexample of a power supply terminal of the present invention, 101 denotesa current source, and 102 denotes a frequency control terminal.Reference numerals 103 a and 103 b denote oscillation transistors, 104 aand 104 b denote inductors, 105 a, 105 b, 106 a, 106 b, 107 a and 107 bdenote variable capacitance elements utilizing a gate capacitance usedin a CMOS process, and 108 a, 108 b, 109 a, 109 b, 110 a and 110 bdenote DC-cut capacitors as an example of a blocking capacitor of thepresent invention. Reference numerals 111 a, 111 b, 112 a, 112 b, 113 aand 113 b denote high-frequency blocking resistances, and 114 denotesreference voltage generation means.

The inductors 104 a and 104 b are series-connected, where the powersource terminal 100 is connected to a connection point thereof so as toform an inductor circuit of the present invention. The variablecapacitance elements 105 a and 105 b are formed by being connected on adrain side and a source side, and the DC-cut capacitors 108 a and 108 bof interrupting direct currents are series-connected to both sides of aseries circuit of the variable capacitance elements 105 a and 105 b. Theseries circuit (hereafter, referred to as a variable capacitance circuitA) of the variable capacitance elements 105 a and 105 b and the DC-cutcapacitors 108 a and 108 b constitutes the variable capacitance circuitof the present invention. Likewise, the series circuit (hereafter,referred to as a variable capacitance circuit B) of the variablecapacitance elements 106 a and 106 b and the DC-cut capacitors 109 a and109 b and the series circuit (hereafter, referred to as a variablecapacitance circuit C) of the variable capacitance elements 107 a and107 b and the DC-cut capacitors 110 a and 110 b constitute the variablecapacitance circuits of the present invention. The oscillationtransistors 103 a and 103 b constitute a negative resistance circuit ofthe present invention. And the above inductor circuit, variablecapacitance circuits A, B and C and the negative resistance circuit ofthe present invention are mutually connected in parallel.

The frequency control terminal 102 is connected to the connection pointof the variable capacitance elements 105 a and 105 b in the variablecapacitance circuit A, the connection point of the variable capacitanceelements 106 a and 106 b in the variable capacitance circuit B, and theconnection point of the variable capacitance elements 107 a and 107 b inthe variable capacitance circuit C, that is, to the drain-source side asan example of one terminal of the variable capacitance element of thepresent invention.

The gate sides of the variable capacitance elements 105 a and 105 b ofthe variable capacitance circuit A are connected to one output of thereference voltage generation means 114 via the resistors 111 a and 111 brespectively. The gate sides of the variable capacitance elements 106 aand 106 b of the variable capacitance circuit B are connected to anotheroutput of the reference voltage generation means 114 via the resistors112 a and 112 b respectively. The gate sides of the variable capacitanceelements 107 a and 107 b of the variable capacitance circuit C areconnected to another output of the reference voltage generation means114 via the resistors 113 a and 113 b respectively.

FIG. 3 shows the configuration example of the reference voltagegeneration means 114. In FIG. 3, reference numeral 120 denotes the powersource terminal, Ra to Rd denote the resistors, and 122 a to 122 cdenote grounded capacitive elements. Reference numeral 121 a denotes anoutput terminal of outputting Vref as an example of a predeterminedreference voltage of the present invention. Reference numeral 121 bdenotes the output terminal of outputting Vref−Vd as another example ofthe predetermined reference voltage of the present invention, and 121 cdenotes the output terminal of outputting Vref−2Vd as a further exampleof the predetermined reference voltage of the present invention.

Hereafter, operation of the voltage-controlled oscillator according tothe first embodiment will be described by referring to FIG. 3.

As for the reference voltage generation means 114 in FIG. 3, it will berepresented as follows if the voltage applied to the power sourceterminal 120 is Vcc and a voltage of signals outputted from the outputterminals 121 a to 121 c is Rb=Rc.Vref=Vcc·(Rb+Rc+Rd)/(Ra+Rb+Rc+Rd)Vref−Vd=Vcc·(Rc+Rd)/(Ra+Rb+Rc+Rd)Vref−2Vd=Vcc·(Rd)/(Ra+Rb+Rc+Rd)  [Formula 1]It is possible, by appropriately selecting values of Ra to Rd, togenerate a reference signal having a desired voltage difference from thereference voltage generation means 114.

In FIG. 1, it is assumed that an inductance of the inductors 104 a and104 b is L, a capacitance value of the variable capacitance elements 105a and 105 b is C1, the capacitance value of the variable capacitanceelements 106 a and 106 b is C2, the capacitance value of the variablecapacitance elements 107 a and 107 b is C3, and the capacitance value ofthe DC-cut capacitors 108 a, 108 b, 109 a, 109 b, 110 a and 110 b is C0.In this case, a resonance frequency f0 of a parallel resonant circuitconstituted by connecting the inductor circuit and variable capacitancecircuits A, B and C in parallel is represented as follows.f0=1/(2π(2L·C′/2)^(1/2))=1/(2π(L·C′)^(1/2))C′=C1·C0/(C1+C0)+C2·C0/(C2+C0)+C3·C0/(C3+C0)  [Formula 2]

Three reference voltages different by a voltage difference Vd areoutputted from the reference voltage generation means 114 as describedabove. They are added to the gate side of the variable capacitanceelements 105 a and 105 b of the variable capacitance circuit A, the gateside of the variable capacitance elements 106 a and 106 b of thevariable capacitance circuit B, and the gate side of the variablecapacitance elements 107 a and 107 b of the variable capacitance circuitC respectively.

A control voltage of feedback-controlling an oscillation frequency isinputted from the frequency control terminal 102 to the drain-sourceside of each variable capacitance element of the variable capacitancecircuits A, B and C.

If the capacitance of each variable capacitance element changes in theneighborhood of a control voltage Vth when the reference voltage isVref, the capacitance values of the variable capacitance circuits A, Band C change against a frequency control voltage as in FIG. 2( a).Therefore, a total capacitance of the variable capacitance circuits A, Band C slowly increases against the control voltage as shown in FIG. 2(b), and as a result, change in the oscillation frequency can be renderedmoderate over a wide range of the control voltage. The control voltagein this case is the voltage difference between each reference voltageand the control voltage.

According to the present invention, it is possible to curb degradationof a phase noise characteristic because no additional circuit such as alevel shift circuit is inserted between the frequency control terminal102 and each variable capacitance circuit. As the voltage applied to thepower source terminal 100 and the reference voltage applied to thevariable capacitance elements 105 a, 105 b, 106 a, 106 b, 107 a and 107b are separated by the DC-cut capacitors 108 a, 108 b, 109 a, 109 b, 110a and 110 b so that it also has an effect of curbing the change in thefrequency due to the variation in the power supply voltage.

The grounded capacitive elements 122 a to 122 c are bypass capacitiveelements for the sake of suppressing noise outputted from the referencevoltage generation means 114. According to the present invention, evenif the capacitive elements are inserted in parallel into the outputterminals 121 a, 121 b and 121 c, there is no influence on a transientresponse characteristic and a noise band characteristic of the PLLcircuit, so that the capacitive elements of a large capacitance valuecan be inserted. To be more specific, even if the capacitive elements ofa large capacitance are inserted and even if the control voltageinputted from a frequency control terminal 5 changes, response thereofwill not deteriorate. Therefore, it is possible to insert sufficientlylarge capacitive elements necessary to curb the noise. Furthermore, itis possible, as shown in FIG. 4, to constitute the capacitive elementsof the large capacitance indicated by 122 d to 122 f as chip componentsand so on on the outside of a semiconductor so as to have a furthersignificant noise suppression effect.

FIG. 5 is another configuration example of the reference voltagegeneration means 114, and the same portions as those previouslydescribed are given the same symbols. In FIG. 5, reference numerals 124a to 124 c denote transistors.

If the voltage between a base and an emitter of the transistors 124 a to124 c is Vbe, collectors and bases of the transistors 124 a to 124 c canbe connected to operate as diodes, and so the voltage difference perstage of diodes can be approximately Vbe.

Therefore, it can be set up as follows.Vref=3VbeVref−Vd=2VbeVref−2Vd=Vbe  [Formula 3]

As for the above circuit, even if a power supply voltage changes, thevoltage difference per stage of diodes is Vbe and it hardly changes.Therefore, it is possible to stabilize the voltage difference of thesignals outputted from the reference voltage generation means 114 so asto realize the oscillator which is hardly influenced by variation in thepower supply voltage.

FIG. 6 is another configuration example of the reference voltagegeneration means 114, and the same portions as those previouslydescribed are given the same symbols. In FIG. 6, reference numeral 130denotes the transistor, 131 denotes the resistor, and 132 denotes thecapacitive element, and the portion surrounded by a broken lineconstitutes an active filter. The resistor 131 series-connected to thebase of the transistor 130 and the grounded capacitive element 132constitute an LPF (low-pass filter). Here, the variable capacitanceelement to be used is designed to satisfy [Formula 5] in [Formula 4] inthe case where a threshold voltage is Vth, the power supply voltage isVcc, and the base-emitter voltage of the transistor is Vbe.Vref=Vcc−Vbe  [Formula 4]Vth<Vcc  [Formula 5 ]

According to this configuration, it is possible to eliminate the noisefrom a power supply with a ripple filter so as to curb the variation inthe reference voltage due to the noise from the power supply compared tothe configuration in FIG. 5.

As shown in FIG. 7, it is also possible, as a further configurationexample of the reference voltage generation means 114, to connect theterminal 121 a to the base of the transistor 130. The voltage of theterminal 121 a becomes lower than the power supply voltage by Vbe in theaforementioned configuration in FIG. 6, whereas the voltage of theterminal 121 a can be set in the neighborhood of the power supplyvoltage in the configuration shown in FIG. 7. Therefore, it is possibleto set the reference voltage in the range of 0V to the power supplyvoltage.

According to this embodiment, the difference between the referencevoltage outputted from the output terminal 121 a of the referencevoltage generation means 114 and the reference voltage outputted fromthe output terminal 121 b and the difference between the referencevoltage outputted from the output terminal 121 b and the referencevoltage outputted from the output terminal 121 c are fixed. However, itgoes without saying that different voltage differences may be given. Inthat case, it should be Rb≠Rc as to the circuit in FIG. 3 for instance.

The above describes the example in which there are three variablecapacitance circuits connected in parallel and three reference voltagesare outputted from the reference voltage generation means 114. However,it is not necessarily limited to three, but may be two in parallel orfour or more in parallel in the configuration.

In that case, there are n (n is two or more) pieces of the variablecapacitance circuit as described above, and n pieces of differentreference voltages are outputted from the reference voltage generationmeans 114 so as to be inputted to the gate side of each variablecapacitance element of the n pieces of the variable capacitance circuit.And the control voltage is inputted to the drain-source side of eachvariable capacitance element of the n pieces of the variable capacitancecircuit, so that the n pieces of output sides of the reference voltagegeneration means 114 should be grounded via the capacitive elementsrespectively.

In that case, when the n pieces of variable capacitance circuits arearranged in decreasing order of reference voltages inputted to said npieces of variable capacitance circuits, the difference between thevoltage inputted to the gate side of an m-th (m is between 2 and n)variable capacitance circuit and the voltage inputted to the gate sideof an m−1-th variable capacitance circuit should be Vd.

The reference voltage generation means 114 in that case should have theconfiguration wherein, in the example shown in FIG. 3 for instance, n+1pieces of resistors are series-connected, and the power supply voltagesupplied from the power source terminal 120 is divided as n pieces ofreference voltage by the n+1 pieces of resistor so as to be outputted.

In the case where the reference voltage generation means is the exampleshown in FIG. 5 or 6, it should have the configuration wherein n piecesof diodes or transistors are series-connected, and the power supplyvoltage is divided as n pieces of reference voltage. In that case, thesame effect as described above can be obtained.

The above describes that the difference between the voltage inputted tothe m-th variable capacitance circuit and that inputted to the m−1-thvariable capacitance circuit is Vd. However, the difference between thevoltages may be different values, and the same effect as described abovecan be obtained in that case.

The above describes that n pieces of different reference voltages areoutputted from the reference voltage generation means 114 to be inputtedto the gate side of each variable capacitance element of the n pieces ofthe variable capacitance circuit. However, it may also be theconfiguration wherein n pieces of reference voltages are outputted fromthe reference voltage generation means 114 to be inputted to the gateside of each variable capacitance element of then pieces of variablecapacitance circuit, and at least two of the n pieces of referencevoltages are different voltages. In that case, the same effect asdescribed above can be obtained.

(Second Embodiment)

FIG. 8 shows a configuration of the voltage-controlled oscillatoraccording to a second embodiment of the present invention, in which thesame portions as those previously described are given the same symbolsand a description thereof will be omitted.

As for the voltage-controlled oscillator according to the firstembodiment, there are the cases where there is a frequency domain inwhich the change in the capacitance of the variable capacitance elementgets saturated even if the control voltage is changed. Thevoltage-controlled oscillator according to this embodiment relates tothe voltage-controlled oscillator capable of changing the capacitance ofthe variable capacitance circuit according to a frequency band.

In FIG. 8, reference numerals 141 a and 141 b denote frequency bandsetting variable capacitance elements, 144 a, 144 b and 144 c denote thevariable capacitance circuits, and 140 denotes a frequency band settingterminal. The variable capacitance elements 141 a and 141 b are mutuallyconnected on the drain-source side to form the series circuit,constituting a first variable capacitance circuit of the presentinvention. The frequency band setting terminal 140 is connected to theconnection point on the drain-source side of the first variablecapacitance circuit.

FIG. 9 shows an internal configuration of the variable capacitancecircuits indicated by 144 a to 144 c. In FIG. 9, reference numerals 200a and 200 b denote connection terminals of connecting the first variablecapacitance circuit and the variable capacitance circuits 144 a to 144 cin parallel. Here, a terminal 201 is connected to the reference voltagegeneration means 114 in FIG. 8, a terminal 202 is connected to thefrequency control terminal 102, and a terminal 203 is connected to thefrequency band setting terminal 140.

Reference numerals 210 a, 210 b, 211 a and 211 b denote the variablecapacitance elements, 212 a, 212 b, 213 a and 213 b denote the DC-cutcapacitors, 214 a, 214 b, 215 a and 215 b denote high-frequency blockingresistors, and 216 denotes a switch.

The variable capacitance elements 210 a and 210 b are mutually connectedon the drain-source side, and the DC-cut capacitors 212 a and 212 b ofinterrupting the direct currents are series-connected to both ends ofthe series circuit of the variable capacitance elements 210 a and 210 b(hereafter, the series circuit formed by the variable capacitanceelements 210 a and 210 b and the DC-cut capacitors 212 a and 212 b isreferred to as a second variable capacitance circuit).

The variable capacitance elements 211 a and 211 b are mutually connectedon the drain-source side, and the DC-cut capacitors 213 a and 213 b ofinterrupting the direct currents are series-connected to both ends ofthe series-circuit of the variable capacitance elements 211 a and 211 b(hereafter, the series circuit formed by the variable capacitanceelements 211 a and 211 band the DC-cut capacitors 213 a and 213 b isreferred to as a third variable capacitance circuit).

The second variable capacitance circuit and the third variablecapacitance circuit are connected in parallel, and are connected to theconnection terminals 200 a and 200 b. And the gate side of the secondvariable capacitance circuit and the gate side of the third variablecapacitance circuit are connected to the reference voltage generationmeans 114 via the resistors 214 a and 214 b and the resistors 215 a and215 b respectively.

The drain-source connection side of the second variable capacitancecircuit is connected to the frequency control terminal 202, and thedrain-source connection side of the third variable capacitance circuitis connected to the output side of the switch 216. One terminal on theinput side of the switch 216 is connected to the frequency controlterminal 202, and the other terminal thereon is connected to the powersupply.

The switch 216 is connected with the control terminal 203, and thecontrol terminal 203 is connected with the frequency band settingterminal 140. And the switch 216 has the configuration wherein one sideof the input side of the switch 216 and the other side thereof areswitched according to a signal outputted from the frequency band settingterminal 140.

Hereafter, the operation of the present invention will be described byreferring to FIGS. 8 and 9.

The voltage-controlled oscillator according to the present inventionapplies a ground voltage or the power supply voltage to the frequencyband setting terminal 140 on starting the operation and thereby changesthe capacitance of the variable capacitance elements 141 a and 141 b ofthe first variable capacitance circuit so as to select the frequencyband to be used. Thereafter, it fine-tunes the oscillation frequencywith the control voltage applied to the frequency control terminal 102.The method of switching the frequency band is already described inJapanese Patent Laid-Open No. 2001-196853. However, as shown in FIG. 10(a), the method in the past has a problem that the higher the frequencyband to be used is, the more abrupt the change in the frequency againstthe control voltage becomes. The reason for this is that, if thefrequency band becomes higher, the capacitance values of the frequencyband setting variable capacitance elements 141 a and 141 b becomesmaller so that, as a result, the capacitance values of the variablecapacitance circuits 144 a to 144 c become relatively larger against allthe parallel capacitance values constituting a resonant circuit.

According to the present invention, the variable capacitance circuitsindicated by 144 a to 144 c in FIG. 8 are constituted as shown in FIG.9, and the capacitance values of the variable capacitance circuits 144 ato 144 c are changed according to the frequency band so as to change thevariable range of the capacitance. To be more specific, in FIG. 9, thevariable capacitance elements 210 a and 210 b are directly connected tothe frequency control terminal 202, and the variable capacitanceelements 211 a and 211 b are selectively connected to the frequencycontrol terminal 202 or the power supply voltage by the switch 216. Thevoltage applied to the control terminal 203 of the switch 216 isswitched synchronizing with the frequency band setting terminal 140. Tobe more specific, in the case where the frequency band on the lowfrequency side is used, the input side of the switch 216 is connected tothe frequency control terminal 202, and is connected to the power supplyvoltage side in the case where the frequency band on the high frequencyside is used. Therefore, it is possible, by appropriately setting thecapacitance values of the variable capacitance elements 210 a, 210 b,211 a and 211 b, to substantially fix the change in the frequencyagainst the control voltage even in the case of switching to a pluralityof bands as shown in FIG. 10( b).

FIG. 11 shows another configuration example of the variable capacitorsindicated by 144 a to 144 c in FIG. 8, and the same portions as thecircuit shown in FIG. 9 are given the same symbols and a descriptionthereof will be omitted.

In FIG. 11, reference numeral 220 a and 220 b denote the DC-cutcapacitors, and 221 a and 221 b denote the switches. The switch 221 aand the capacitor 220 a are series-connected, and they are connected sothat the capacitor 220 a is connected to a capacitor 212 a in parallelwhen the switch 221 a is in a connection state. Likewise, the switch 221b and the capacitor 220 b are series-connected, and they are connectedso that the capacitor 220 b is connected to a capacitor 212 b inparallel when the switch 221 b is in a connected state. And the switches221 a and 221 b are connected to the control terminal 203, and areopened and closed according to the signal outputted from the frequencyband setting terminal 140.

Hereafter, the operation in the case of using the circuit shown in FIG.11 as the variable capacitance circuit shown in FIG. 8 will be describedby referring to FIGS. 8 and 11.

The control terminal 203 of the switches 221 a and 221 b is connected tothe frequency band setting terminal 140. In the case where the frequencyband on the low frequency side is used, the switches 221 a and 221 b areset in the connected state. And in the case where the frequency band onthe high frequency side is used, the switches 221 a and 221 b are set inan interrupted state.

Now, it is assumed that the capacitance value of the variablecapacitance elements 210 a and 210 b is C0, the capacitance value of theDC-cut capacitors 212 a and 212 b is C1, and the capacitance value ofthe DC-cut capacitors 220 a and 220 b is C2. In this case, a totalcapacitance value C of 210 a, 212 a and 220 a or 210 b, 212 b and 220 bis represented as follows when the switches 221 a and 221 b are in theconnected state.C=C0·(C1+C2)/(C0+C1+C2)  [Formula 6]And it is represented as follows when the switches 221 a and 221 b arein the interrupted state.C=C0·C1/(C0+C1)  [Formula 7]And it is possible to change the capacitance value of the entirevariable capacitance circuit shown in FIG. 11 according to the operationof the switches 221 a and 221 b. To be more specific, it is possible, byappropriately lowering the variable range of the capacitance of thevariable capacitance elements 210 a, 210 b, 211 a and 211 b according toreduction in the capacitance of the first variable capacitance circuit,to substantially fix the change in the frequency against the controlvoltage even in the case of switching to the plurality of bands as shownin FIG. 10( b). To be more specific, minute influence of the noise overthe control voltage can be curbed even in the case of using the highfrequency band.

According to this embodiment, the case of two frequency bands has beendescribed. It is possible, however, to support three or more frequencybands with the same configuration. In that case, of the plurality ofvoltages, the voltage corresponding to the frequency band is inputted tothe first variable capacitance circuit and the variable capacitancecircuits 144 a to 144 c. Here, in the case where the circuit shown inFIG. 9 is used as the variable capacitance circuits 144 a to 144 c,there is a thinkable configuration wherein a different variablecapacitance circuit of the same configuration is further connected inparallel in addition to the second variable capacitance circuit andthird variable capacitance circuit, and the power supply voltage isinputted according to the voltage inputted from the frequency bandsetting terminal 140 as with the second variable capacitance circuit.

In the case where the circuit shown in FIG. 11 is used as the variablecapacitance circuits 144 a to 144 c, there is a thinkable configurationwherein the series circuit of the switch and capacitor is furtherconnected in parallel to the capacitors 212 a and 212 b, and thecapacitor is further connected according to the voltage inputted fromthe frequency band setting terminal 140.

The voltage-controlled oscillator according to this embodiment may alsohave the configuration wherein the n pieces of variable capacitancecircuit exist, at least one of the n pieces of variable capacitancecircuit has at least one variable capacitance element, and of theplurality of voltages, the voltage according to the frequency to be usedis inputted instead of the control voltage to the terminal on thedrain-source side of this at least one variable capacitance element sothat the variable range of the capacitance of at least one of the npieces of variable capacitance circuit is lowered according to thereduction in the capacitance of the variable capacitance element of thefirst variable capacitance circuit.

The voltage-controlled oscillator according to this embodiment may alsohave the configuration wherein the DC-cut capacitor is comprised of twoor more capacitors connected via the switch, and the capacitance of theDC-cut capacitor is reduced as the above switch is on or off accordingto the frequency to be used so that the variable range of thecapacitance of at least one of the n pieces of variable capacitancecircuit is lowered.

This embodiment has been described by taking the variable capacitancecircuit shown in FIG. 9 or 11 as an example so far. However, the npieces of variable capacitance circuit is not limited to the exampleshown in FIG. 9 or 11 but may be the variable capacitance of anotherconfiguration. To be more specific, the n pieces of variable capacitancecircuit should have the configuration wherein the power supply voltageor the predetermined reference voltage is inputted to some terminals ofthe variable capacitance elements of the n pieces of variablecapacitance circuit, the control voltage of feedback-controlling theoscillation frequency is inputted to the other terminals of the variablecapacitance elements of said n pieces of variable capacitance circuits,and when the capacitance of the variable capacitance element of thefirst variable capacitance circuit becomes lower than the predeterminedvalue, the variable range of the capacitance of at least one of the npieces of variable capacitance circuit is lowered. In that case, thesame effect as described above can be obtained.

According to the above description of this embodiment, the capacitiveelements 122 a, 122 b and 122 c are connected to the output side of thereference voltage generation means 114. However, the voltage-controlledoscillator according to this embodiment does not necessarily require thecapacitive elements 122 a, 122 b and 122 c. In that case, the sameeffect as described above can be obtained in that it is not influencedby the noise from the power supply voltage or the reference voltage.

According to the above description of the first and second embodiments,the reference voltages outputted from the reference voltage generationmeans 114 are inputted to the gate side of each variable capacitanceelement of the variable capacitance circuits and the control voltagesare inputted to the drain-source side of each variable capacitanceelement of the variable capacitance circuits. However, it may also bethe configuration wherein the control voltages are inputted to the gateside of each variable capacitance element of the variable capacitancecircuits and the reference voltages are inputted to the drain-sourceside of each variable capacitance element of the variable capacitancecircuits.

According to the above description, the variable capacitance elementsare those utilizing the gate capacitance used in the CMOS process.However, the variable capacitance elements may be another type ofvariable capacitance elements. In that case, the reference voltagesshould be inputted to one side of each variable capacitance element andthe control voltages should be inputted to the other side thereof, andthe same effect as described above can be obtained.

(Third Embodiment)

The voltage-controlled oscillator according to a third embodiment of thepresent invention will be described next by referring to FIGS. 17 to 20.

FIG. 17 shows a configuration example of the voltage-controlledoscillator according to this embodiment. The components in common withthe first and second embodiments are given the same reference numerals,and a description thereof will be omitted. The voltage-controlledoscillator shown in FIG. 17 has variable capacitance elements 305 a and305 b series-connected to constitute the variable capacitance circuit A,and has variable capacitance elements 306 a and 306 b series-connectedto constitute the variable capacitance circuit B. The gate sides of thevariable capacitance elements 305 a, 305 b, 306 a and 306 b areconnected to the power supply voltage side without involving the DC-cutcapacitors 110 a and 110 b. The reference voltage generation means 114is not used in this embodiment.

In the circuit shown in FIG. 17, the variable capacitance elements usedin the variable capacitance circuit A and those used in the variablecapacitance circuit B have different characteristics against thevoltage. FIG. 18 illustrates such a difference in the characteristics.For instance, a curve A shown in FIG. 18 indicates the characteristic ofthe variable capacitance elements 305 a and 305 b used in the variablecapacitance circuit A, and a curve B indicates the characteristic of thevariable capacitance elements 306 a and 306 b used in the variablecapacitance circuit B. Such a difference in the characteristics of thevariable capacitance elements can be realized, for instance, bydifferentiating impurity concentrations of the wells of the variablecapacitance elements.

As shown in FIG. 17, the variable capacitance circuits A and B areconnected in parallel so that the characteristic of the entire circuitbecomes moderate as indicated by the broken line in FIG. 18. Therefore,the voltage-controlled oscillator according to this embodiment has theeffect of curbing phase noise as with the voltage-controlled oscillatoraccording to the first embodiment.

FIG. 19 shows a configuration example in the case where variablecapacitance elements 405 a and 405 b utilizing an n-channel MOS are usedin the variable capacitance circuit A and variable capacitance elements406 a and 406 b utilizing a p-channel MOS are used in the variablecapacitance circuit B (reversal thereof is also possible). As thecharacteristic of the capacitance against the voltage is reverse betweenthe n-channel MOS and the p-channel MOS, the variable capacitanceelements are connected in reverse directions in the variable capacitancecircuits A and B respectively as shown in FIG. 19 (the variablecapacitance elements 405 a and 406 a for instance). The same effect asdescribed above can be obtained in such a configuration.

FIG. 20 shows a configuration example in the case where variablecapacitance elements 505 a and 505 b utilizing an inversion MOS ofhaving the capacitance formed between the gate and the drain-source areused as the variable capacitance elements in the variable capacitancecircuit A, and variable capacitance elements 506 a and 506 b utilizingan accumulation MOS of having the capacitance formed between the gateand the well are used as the variable capacitance elements in thevariable capacitance circuit B (reversal thereof is also possible). Asthe characteristic of capacitance against the voltage is reverse betweenthe inversion MOS and the accumulation MOS, the variable capacitanceelements are connected in reverse directions respectively as shown inFIG. 20 (the variable capacitance elements 505 a and 506 a forinstance). Because of such a configuration, the characteristic of theentire circuit as to the capacitance against the voltage becomes moremoderate as indicated by the broken line in FIG. 21 so that the sameeffect as described above can be obtained.

According to this embodiment, the examples have been described by usingtwo variable capacitance circuits. However, it may be the configurationhaving the variable capacitance circuits further connected thereto. Inthat case, it may be the configuration wherein the characteristics ofthe variable capacitance elements in each variable capacitance circuitare different or may also be the configuration wherein thecharacteristics of the variable capacitance elements in at least twovariable capacitance circuits are different. Even in such a case, thesame effect as described above can be obtained if the characteristic ofthe entire circuit as to the capacitance against the voltage becomesmore moderate.

It is also thinkable that the variable capacitance circuits shown inFIGS. 17 to 20 are combined. Even in such a case, the same effect asdescribed above can be obtained if the characteristic of the entirecircuit as to the capacitance against the voltage becomes more moderate.

According to the above description, MOS transistors are used as theoscillation transistors. However, bipolar transistors may also be used.

The scope of the present invention also includes a radio communicationapparatus having the voltage-controlled oscillator according to thefirst and second embodiments and also having a PLL circuit of outputtingthe signal of a target frequency.

As described above, it is possible, according to the present invention,to moderate the change in the oscillation frequency against the controlvoltage of the voltage-controlled oscillator even in the case of usingthe variable capacitance elements of low linearity. Therefore, it ispossible to maintain constant transient response characteristic andnoise band characteristic of the PLL circuit over a wide band andconnect a large-capacitance bypass capacitor in parallel to the signaloutput from the reference voltage generation means so as to realize theoscillator of a good phase noise characteristic.

According to the above description, the power source terminal 100 isconnected to the connection point of the inductors 104 a and 104 b.However, it is not limited thereto, and the configuration shown in FIG.16 is also thinkable.

The voltage-controlled oscillator and voltage-controlled oscillationmethod according to the present invention can curb the influence ofvariation or the noise in the power supply voltage or curb degradationof the phase noise characteristic so that they are useful for the radiocommunication apparatus and so on.

1. A voltage-controlled oscillator comprising: an inductor circuithaving an inductor; n pieces (n is two or more) of variable capacitancecircuit having a variable capacitance element and having blockingcapacitors of interrupting a direct current at both ends thereof, said npieces of variable capacitance circuit including first, second and thirdvariable capacitance circuits, a negative resistance circuit, andreference voltage generation means having m pieces of resistanceconnected in series between a power supply voltate and a groundingpotential wherein: m is at least two; said inductor circuit, said npieces of variable capacitance circuit, and said negative resistancecircuit are connected in parallel; said reference voltage generationmeans dividing the power supply voltage by use of said m pieces ofresistance to supply m pieces of reference voltage to one terminal ofthe variable capacitance elements of said n pieces of variablecapacitance circuit from m pieces of connecting point between adjacentresistances of said m pieces of resistance; a control voltage offeedback-controlling an oscillation frequency is inputted to the otherterminals of the variable capacitance elements of said n pieces ofvariable capacitance circuits; said n pieces of variable capacitancecircuit are connected in parallel with each other so that eachcapacitance variation characteristic against said control voltageinputted to said variable capacitance circuit has the same-directedcharacteristic; said control voltage inputted to the other terminals ofthe variable capacitance elements of said n pieces of variablecapacitance circuit is the same voltage; in said reference voltagegeneration means, a first resistance and a second resistance connectedsuccessively are configured so that a first reference voltage issupplied from a connection point positioned between said firstresistance and said power supply voltage, a second reference voltage issupplied from the connection point between said first resistance andsaid second resistance, a third reference voltage is supplied from aconnection point positioned between said second resistance and saidgrounding potential, and the resistance values of said first resistanceand said second resistance are different than each other, of said npieces of variable capacitance circuits, said first reference voltage isinputted to one of the terminals of said first variable capacitancecircuit, said second reference voltage is inputted to one of theterminals of said second variable capacitance circuit, and said thirdreference voltage is inputted to one of the terminals of said secondvariable capacitance circuit; said first reference voltage, said secondreference voltage, and said third reference voltage are fixed andrespectively; and a first difference between said first referencevoltage and said second reference voltage and a second differencebetween said second reference voltage and said third reference voltageare each different than each other wherein the linearity of a resonantfrequency characteristic of the voltage-controlled oscillator isimproved.
 2. A voltage-controlled oscillator according to claim 1,further having connected in parallel thereto a frequency band settingvariable capacitance circuit having a variable capacitance element,having a power supply voltage inputted to one terminal of said variablecapacitance element and having a voltage according to a frequency bandto be used of a plurality of voltages inputted to the other terminal ofsaid variable capacitance element so as to change a capacitance of saidvariable capacitance element, wherein, when the capacitance of thevariable capacitance element of said first variable capacitance circuitbecomes less than a predetermined value, a variable range is reduced asto the capacitance of at least one variable capacitance circuit of saidn pieces of variable capacitance circuit.
 3. The voltage-controlledoscillator according to claim 2, wherein at least one variablecapacitance circuit of said n pieces of variable capacitance circuit hasthe variable capacitance element connected in parallel to said variablecapacitance element, and the voltage according to a frequency to be usedof said plurality of voltages is inputted to the other terminal of oneof said variable capacitance elements instead of said control voltage sothat the variable range is reduced as to the capacitance of at least onevariable capacitance circuit of said n pieces of variable capacitancecircuit according to the reduction in the capacitance of the variablecapacitance element of said frequency band setting variable capacitancecircuit.
 4. The voltage-controlled oscillator according to claim 2,wherein said blocking capacitor is comprised of two or more capacitorsconnected via a switch, and said switch is off or on according to thefrequency to be used so that the capacitance of said blocking capacitorsis reduced and a variable range of the capacitance of at least one ofsaid n pieces of variable capacitance circuit is reduced.
 5. Thevoltage-controlled oscillator according to claim 1, wherein at least oneof said first variable capacitance circuit, said second variablecapacitance circuit, and said third variable capacitance circuit is anaccumulation MOS.
 6. A radio communication apparatus having a PLLcircuit of outputting a signal of a target frequency, said PLL circuithaving the voltage-controlled oscillator according to claim 1.